Quite simply you cannot. It is widely stated you would need to be at a height of at least 40,000 ft to get even a hint of curvature if earth were round. Commercial aircraft are not allowed to fly this high. They are only allowed to fly just under this altitude. 36,000ft might be typical. In addition, the windows on commercial aircraft are small and heavily curved. Even if they flew high enough for a person to see curvature, it would still not be visible to passengers.

148) Quoting “Earth Not a Globe!” by Samuel Rowbotham, “It is found by observation that the stars come to the meridian about four minutes earlier every twenty-four hours than the sun, taking the solar time as the standard. This makes 120 minutes every thirty days, and twenty-four hours in the year. Hence all the constellations have passed before or in advance of the sun in that time. This is the simple fact as observed in nature, but the theory of rotundity and motion on axes and in an orbit has no place for it. Visible truth must be ignored, because this theory stands in the way, and prevents its votaries from understanding it.”

Newtonian philosophers teach us that the Moon goes round: the Earth from west to east. But observation - man's most certain mode of gaining knowledge - shows us that the Moon never ceases to move in the opposite direction - from east to west. Since, then, we know that nothing can possibly move in two, opposite directions at the same time, it is a proof that the thing is a big blunder; and, in short, it is a proof that the Earth is not a globe.

96.) If we refer to the diagram in "Cornell's Geography," page 4, and notice the ship in its position the most remote from the observer, we shall find that, though it is about 4,000 miles away, it is the same size as the ship that is nearest to him, distant about 700 miles! This a an illustration of the way in which astronomers ignore the laws of perspective. This course is necessary, or they would be compelled to lay bare the fallacy of their dogmas. In short, there is, in this matter, a proof that the Earth is not a globe.

If the earth is flat we are held hostage to one of the biggest hoaxes of the last 500 hundred years of human existence. Every day I see evidence of the flat earth. Just look at how at time the clouds are not moving at all --completely still. If the earth was spinning at 1600 klms per hour --how could it be that at times the clouds are completely still not moving in the sky at all.(how could the clouds be moving at the same speed as the rotating earth?)When you look at a long distant horizon --from right to left --there is no curviture in the earth whatsoever. If you try to point this out to people they will think you have lost your mind.. we have been brainwashed to believe a lie which is very difficult to prove to the world populous that there is a flat earth. The deception is wrapped in scientific mumbo jumbo which is very easy to accept as it is backed up by a lot of scientific explanation--if you want to take the easy way out--you just accept that the earth is a globe spinning at 1600 klms per hour . But how does all the water of the seas stay stuck to the planet when the earth is spinning around at 1600 klms per hour? (gravity cannot hold water upside down--water will flow to the lowest point--how can that happen on a spinning globe?)

153) Quoting Reverend Thomas Milner’s “Atlas of Physical Geography,” we find that, “Vast areas exhibit a perfectly dead level, scarcely a rise existing through 1,500 miles from the Carpathians to the Urals. South of the Baltic the country is so flat that a prevailing north wind will drive the waters of the Stattiner Haf into the mouth of the Oder, and give the river a backward flow 30 or 40 miles. The plains of Venezuela and New Granada, in South America chiefly on the left of the Orinoco, are termed Ilanos, or level fields. Often in the space of 270 square miles the surface does not vary a single foot. The Amazon only falls 12 feet in the last 700 miles of its course; the La Plata has only a descent of one thirty-third of an inch a mile.”
2. Another related thing I don’t understand: if the sun and moon are always above the disk of the Earth, why can’t everyone in the world see them at all times? Surely they should always be visible, at least at a low angle. I can’t draw myself any diagram where they are not always visible, but we see that that doesn’t happen. I can’t see how night time happens. Help!
75.) Considerably more than a million Earths would be required to make up a body like the Sun -the astronomers tell us: and more than 53,000 suns would be wanted to equal the cubic contents of the star Vega. And Vega is a "small star!" And there are countless millions of these stars! And it takes 30,000,000 years for the light of some of those stars to reach us at 12,000,000 miles in a minute! And, says Mr. Proctor, "I think a moderate estimate of the age of the Earth would be 500,000,000 years! "Its weight," says the same individual, "is 6,000,000,000,000,000,000,060 tons!" Now, since no human being is able to comprehend these things, the giving of them to the world is an insult – an outrage. And though they have all risen from the one assumption that Earth is a planet, instead of upholding the assumption, they drag it down by the weight of their own absurdity, and leave it lying in the dust – a proof that Earth is not a globe.
163) NASA and other space agencies have been caught time and again with air bubbles forming and floating off in their official “outer-space” footage. Astronauts have also been caught using scuba-space-gear, kicking their legs to move, and astronaut Luca Parmitano even almost drowned when water started filling up his helmet while allegedly on a “space-walk.” It is admitted that astronauts train for their “space-walks” in under-water training facilities like NASA’s “Neutral Buoyancy Lab,” but what is obvious from their “space bubbles,” and other blunders is that all official “space-walk” footage is also fake and filmed under-water.
His first musical stirrings were at the age of eight, when his parents gave him a secondhand radiogram which included a few records left by the previous owner. Among them were Drummin' Man by drumming legend Gene Krupa, and, in Davies's own words, "it hit like a thunderbolt". "I must have played it 2,000 times," he said. "That was it."[5] A friend of the family made Rick a makeshift drum kit out of a biscuit tin, and at the age of 12 he joined the British Railways Staff Association Brass and Silver Jubilee Band as a snare drummer.[6] In an interview in 2002 he said: "As a kid, I used to hear the drums marching along the street in England, in my home town, when there was some kind of parade, and it was the most fantastic sound to me. Then, eventually, I got some drums and I took lessons. I was serious about it... I figured if I could do that – I mean a real drummer, read music and play with big bands, rock bands, classical, Latin, and know what I was going to do – I would be in demand and my life was set... Eventually, I started fiddling with the keyboards, and that seemed to go over better than my drumming, for some reason. So you've gotta go with what people react to." He never had lessons for keyboards, but, according to Betty Davies, "taught himself most of what he knows about music".[5]
56.) The Sun and Moon may often be seen high in the heavens at the same time – the Sun rising in the east and the Moon setting in the west – the Sun's light positively putting the Moon's light out by sheer contrast! If the Newtonian theory were correct, and the moon had her light from the Sun, she ought to be getting more of it when face to face with that luminary – if it were possible for a sphere to act as a reflector all over its face! But as the Moon's light pales before the rising Sun, it is a proof that the theory fails; and is gives us a proof that the Earth is not a globe.
Light travels at a finite speed, with the speed depending upon the medium. With mirages, the medium is air. In air, the speed of light is only slightly less than it is in a vacuum, and the speed of light in air depends upon the temperature of the air. Simply put, the speed of light is slightly greater in warmer air than it is in cooler air. In physics, we usually express this behavior reciprocally as the index of refraction, n:
173) NASA has several alleged photographs of the ball-Earth which show several exact duplicate cloud patterns! The likelihood of having two or three clouds of the exact same shape in the same picture is as likely as finding two or three people with exactly the same fingerprints. In fact it is solid proof that the clouds were copied and pasted in a computer program and that such pictures showing a ball-shaped Earth are fakes.
96.) If we refer to the diagram in "Cornell's Geography," page 4, and notice the ship in its position the most remote from the observer, we shall find that, though it is about 4,000 miles away, it is the same size as the ship that is nearest to him, distant about 700 miles! This a an illustration of the way in which astronomers ignore the laws of perspective. This course is necessary, or they would be compelled to lay bare the fallacy of their dogmas. In short, there is, in this matter, a proof that the Earth is not a globe.

The most commonly accepted explanation of this is that the space agencies of the world are involved in a conspiracy faking space travel and exploration. This likely began during the Cold War's 'Space Race', in which the USSR and USA were obsessed with beating each other into space to the point that each faked their accomplishments in an attempt to keep pace with the other's supposed achievements. Since the end of the Cold War, however, the conspiracy is most likely motivated by greed rather than political gains, and using only some of their funding to continue to fake space travel saves a lot of money to embezzle for themselves.